Conventionally, a vehicle generally has an engine cooling water channel with an engine, a heater core and a radiator located on the engine cooling water channel, and a thermostat monitor for detecting an open failure of a thermostat uses two temperature sensors comprising an outlet-region temperature sensor positioned in the vicinity of an outlet of an in-engine cooling water channel included in the engine cooling water channel, and a radiator-side temperature sensor provided on a radiator-side cooling water channel. Temperatures at two locations (temperature of the engine cooling water flowing out from the in-engine cooling water channel and temperature of the engine cooling water flowing out from the radiator-side temperature sensor) are directly measured respectively by such two temperature sensors, to thereby directly determine a decrease in temperature of the engine cooling water flowing out from the radiator-side cooling water channel.